Fix: create venv in python_installer stage

The Python 3.14 upgrade changed the base image from python_development
(which had a pre-created venv) to wax/ci-base-image (which doesn't).
This caused CI builds to fail with "No such file or directory" when
trying to activate /python_venv/bin/activate.

See: !812

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>